This invention concerns the field of electrical measurements (including the measurement of electrical current flow) accomplished with minimal measured circuit interruption within the small confines of an operational electronic integrated circuit die and with the measured signals being of microwave or millimeter wave or the like electrical frequency.
In the design, fabrication, use and analysis of integrated circuit devices and other electrical assemblies, there often arises a need for quantitative and accurate assessment of the electrical operating conditions imposed on specific elements comprising the complete circuit assembly. Such quantitative information can be of assistance in assuring long operating life of each element in the circuit assembly, in understanding reasons if this life is not achieved and in optimizing such characteristics as element physical size, element attained operating temperature and signal path intercoupling tendencies, for examples.
As a specific example of using the present invention measurement concepts, it is found to be desirable when analyzing monolithic microwave integrated circuits (MMICs) or millimeter wave integrated circuit devices to obtain signal information relevant to actual signal nodes existing within the device--to obtain realistic values for the drain voltage and drain current experienced by a particular transistor of the parallel-connected transistors in a heterojunction field-effect transistor amplifier for example. Often such accurate measurements of transistor drain or gate signal levels will identify a failure-inducing condition, however, such measurements at nodes located within an MMIC device have not heretofore been available.
Notably, for example, the measurements disclosed in the technical paper "Non-Invasive Waveform Probing for Nonlinear Network Analysis," by C. J. Wei, Y. A. Tkachenko and J. C. M. Hwang, which appears in the IEEE MTT-S Int'l Microwave Symp. Dig., May 1993, pp. 1347-1350 have certain similarity to those of the present invention with the significant exception that the measurements of this paper are all accomplished external to the device under test. It is also notable that although this paper discloses, at page 1349, column 1, line 3-4, the use of a non grounded high impedance test probe, several ramifications of such a non grounded probe to the device under test may not have been appreciated and disclosed in this paper.
When the needed integrated circuit information resides in direct current and low frequency electrical signals it is usually possible to perform quantitative measurements even within an integrated circuit die using well-known conventional electrical measurement tools. When the needed measurement involves signals of microwave or millimeter wave or the like electrical frequency, however, the performance of even certain fundamental measurements at nodes of an integrated circuit device is accomplished only with difficulty or is impossible of practical accomplishment because of the technical complexities involved. In many instances the measurement of electrical signal current flow has come within this latter difficult or impossible category.
Non-invasive, internal-node waveform probing is a technique which therefore can be of assistance in measurement and diagnosis of integrated circuits under real operating conditions, however this technique has heretofore been useful primarily in circuits which operate at a frequency below one gigahertz. This non-invasive, internal-node waveform probing has heretofore not extended to MMICs and similar circuits often because of the tendency for the probe employed to perturb the MMIC operation so greatly as to make the measured results of little value. Moreover the often used sampling oscilloscope generates only time-domain information, which makes correction for instrument dispersion by standard frequency-domain calibration techniques difficult. The typical microwave measurement probe also presents a major disturbance or leakage path to a signal node under test, e.g., to a transmission line under test, because both the probe and the transmission line have characteristic impedances near fifty ohms.